1. Field of the Invention
The present invention relates to a method of allocating radio resources in a multi-carrier system, and more particularly, to a method of transmitting a signaling message from a user equipment communicating with a multi-carrier system. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for an OFDM communication system, a DFT-S-OFDM communication system, an OFDMA communication system or a communication system of transmitting data carried by a plurality of subcarriers to maintain orthogonality between a plurality of the subcarriers.
2. Discussion of the Related Art
Generally, OFDM, DFT-S-OFDM (DFT spreading OFDM) and OFDMA systems are used as communication methods in multi-carrier systems, which is explained as follows.
In the following description, OFDM (orthogonal frequency division multiplexing) is explained. A basic principle of OFDM lies in dividing a high-rate data stream into a number of slow-rate data streams and transmitting the slow-rate data streams simultaneously using a plurality of carriers. In this case, each of a plurality of the carriers is called a subcarrier. Since orthogonality exists between a plurality of the carriers of OFDM, even if frequency components of the carriers are mutually overlapped, they can be detected by a receiving end. The high-rate data stream is converted to a plurality of slow-rate data streams by a serial-to-parallel converter, a plurality of the parallel-converted data streams are multiplied by subcarriers, respectively, the multiplied data streams are summed up together, and the corresponding sum is then transmitted to the receiving terminal.
A plurality of the parallel-converted data streams can be transmitted as a plurality of subcarriers by IDFT (Inverse Discrete Fourier Transform). And, the IDFT can be efficiently implemented using IFFT (Inverse Fast Fourier Transform).
As a symbol duration of a subcarrier having a low data rate increases, relative signal dispersion occurring by multi-path delay spreading decreases. And, it is able to reduce inter-symbol interference by inserting a guard interval, which is longer than delay spreading of channel, between OFDM symbols. Moreover, a portion of an OFDM signal is copied and placed at a start portion of symbols in a guard interval. If so, OFDM symbols are cyclically extends to be protected.
DFT-S-OFDM according to a related art is explained as follows.
First of all, the DFT-S-OFDM is called SC-FDMA (Single Carrier-FDMA) as well. The SC-FDMA is the scheme mainly applicable to uplink. In the SC-FDMA, a spreading scheme is first applied in a frequency domain by DFT matrix before generating an OFDM signal, the corresponding result is modulated by the OFDM scheme, and the modulated result is then transmitted.
FIG. 1 shows a configuration of a transmitting end according to DFT-S-OFDM. To explain operations of the related art device, several variables are defined. ‘N’ indicates a number of subcarriers carrying OFDM signal, ‘Nb’ indicates a number of subcarriers for a random user, ‘F’ indicates Discrete Fourier Transform matrix, i.e., DFT matrix, ‘s’ indicates a data symbol vector, ‘x’ indicates a spread vector in frequency domain, and ‘y’ indicates a transmitted OFDM symbol vector in time domain.
In SC-FDMA, data symbol (s) is spread using DFT matrix before being transmitted. This is represented as Formula 1.x=FNb×Nbs  [Formula 1]
In Formula 1, FNb×Nb is an Nb-sized DFT matrix used to spread data symbol (s). Subcarrier mapping is performed on the spread vector (x) by a predetermined subcarrier assigning scheme and a signal to be transmitted to a receiving end is obtained from transforming the corresponding result into a time domain by IDFT module. A signal transmitted to the receiving end is represented as Formula 2.y=FN×N−1x  [Formula 2]
In Formula 2, FN×N−1 is an N-sized DFT matrix used to convert a frequency-domain signal to a time-domain signal. A cyclic prefix is inserted in a signal ‘y’ generated by the above method to be transmitted. And, a method of generating a transmission signal and transmitting the signal to a transmitting terminal in the above manner is called SC-FDMA. And, it is able to control a size of DFT matrix in various ways for specific purposes. For instance, if the size of the DFT matrix is equal to the number of points of IDFT, it is able to reduce PAPR in a transmitting terminal.
OFDMA (Orthogonal Frequency Division Multiple Access) according to a related art is explained as follows.
First of all, OFDMA is a multiple access method for implementing a multiple access in a modulating system using a plurality of orthogonal subcarriers in a manner of providing each user with available subcarriers. In the OFDMA, frequency resources called subcarriers are provided to a plurality of users, respectively. In general, the frequency resources are independently provided to a plurality of the users, respectively not to be overlapped with each other.
A control signal transmitting method in LTE (Long Term Evolution) system is explained as follows.
First of all, unlike the uplink of the conventional CDMA communication system, an uplink of the LTE system uses an access method using a plurality of subcarriers having orthogonality.
In case of a system supporting a circuit type service, if there exists data to be transmitted to the LTE system, a user equipment (UE) makes a request for a call setup to a Node B. If the call setup is achieved by the Node B, the user equipment keeps transmitting the data. Even if there exists no data to be transmitted in this system, the call keeps being maintained. After the call maintained status has passed by, the call is disconnected. For this reason, in case of a service that mainly uses packet type data, system efficiency gets degraded.
So, in case of a system that mainly provides a packet data service, the service is provided using a shared channel shared by all user equipments to enhance system efficiency. In particular, in an orthogonal frequency division access system that transmits packets via the shared channel, orthogonality between data channels transmitted in uplink should be maintained. Through this, transmission efficiency of the system can be raised.
Explained in the following description is a method of transmitting data via the shared channel.
A general scheduled transmission mode is explained.
First of all, a Node B performs scheduling and designates a UE to transmit data according to a result of the scheduling. According to the scheduling result, the Node B exclusively allocates resources in frequency and time domains to be used by the user equipment and then informs the user equipment of the allocated resources. So, it is able to previously prevent collisions between a plurality of user equipments in data transmission. In other words, uplink resources managed by the Node B are shared by a plurality of the user equipments. The scheduling for allocating the uplink shared channel to a plurality of the user equipments under the supervision of the Node B so that the corresponding user equipment transmits data to the Node B. This transmission mode is defined as a scheduled transmission mode.
The scheduled transmission mode is also characterized in that a scheme for maintaining synchronization between user equipments transmitting data in uplink should be used. In particular, in order to maintain orthogonality between signals transmitted from a plurality of user equipments in uplink, signals transmitted from a plurality of the user equipments should be received by the Node B at the same time with error ranging within a cyclic prefix of OFDM.
For this, the Node B should adjust a transmission timing point of each of the user equipments by calculating time information for the signal of each of the user equipments and setting the timing point ahead or behind.
A contention based transmission mode is explained as follows.
First of all, the contention based transmission mode is preferably used for a case that the Node B is unable to designate a user equipment by performing the scheduling in advance or a case that a user equipment should male a transmission arbitrarily without a permission of base station to minimize the time delay attributed to the scheduling.
There should be a clear discrimination between the contention based transmission mode and the scheduled transmission mode in a time or frequency domain.
FIG. 2A to FIG. 2C show methods of discriminating the contention based transmission mode and the scheduled transmission mode from each other. FIG. 2A shows a method of discrimination in a time domain. FIG. 2B shows a method of discrimination in a frequency domain. And, FIG. 2C shows a discriminating method by combination between the time domain discrimination and the frequency domain discrimination.
Consequently, resources transmitted by the scheduled transmission mode or the contention based transmission mode are discriminated based on a specific time domain, a specific frequency domain, or a combination of the specific time and frequency domains.
For a specific frequency-time domain to which the contention based transmission mode is applied, a plurality of the user equipments are free to transmit specific data. Yet, since resource scheduling of the Node B is not carried out in the frequency-time domain, it may happen that a plurality of user equipments may use the same resource at the same time. If a plurality of the user equipments use the same resource simultaneously, the Node B has a problem in recovering all information received from a plurality of the user equipments. In this case, the Node B is able to recover the information transmitted from the user equipment corresponding to a signal having a biggest reception power.
The Node B transmits a acknowledgement signal to the user equipment having transmitted a message detected by the Node B. In this case, the UE failing to receive the acknowledgement (ACK) may attempt a message transmission again via the frequency-time domain for performing the contention based transmission after a specific time has passed by. In particular, a basic operation of the contention based transmission is carried out in a manner of sending a signal to the Node B, waiting for the ACK, and attempting a retransmission in case of failing to receive the AC.
A representative example of the contention based transmission is the data transmission using RACH (Random Access Channel). The transmission system via the RACH is explained as follows.
First of all, a user equipment (UE) obtains timing information of downlink and receives system information transmitted via downlink. In this case, the system information is broadcasted in general. The user equipment obtains information for a time interval and frequency domain available for a contention based transmission via the broadcasted signal. Yet, the Node B is unable to know the presence of the user equipment. So, the user equipment applies a specific code to a specific sequence called a preamble and then transmits the preamble to the Node B to inform of the presence of the corresponding user equipment.
Subsequently, the Node B detects the preamble and then transmits ACK (acknowledgement) to the user equipment using a specific code assigned to the code applied to the preamble.
Through the above process, the data transmission via RACH is executed. The preamble may include other informations (e.g., resource allocation request UE ID, etc.) except the preamble sequence including the specific code. Moreover, it is able to transmit the preamble and the information for temporary UE ID) or the like via the RACH.
Yet, the above-explained preamble transmission via the RACH has a danger of data collision as well as a danger attributed to the contention. The data collision according to the data transmission via the RACH is explained as follows.
First of all, if a plurality of user equipments attempt accesses to a Node B suing the same preamble code at the same time, the collision may take place. The Node B is capable of detecting an access attempt transmitted from one of a plurality of the user equipments only and transmitting ACK (acknowledgement) in response to the access attempt. Yet, in viewpoint of the user equipment, the corresponding user equipment regards the ACK as transmitted to itself, whereby the collision takes place. So, a collision detection process for preventing the collision should be executed in a physical or upper layer.
In the contention based transmission mode, a transmission may be basically carried out while reception synchronization occurs between a plurality of user equipments. And, it is also possible to assume the other case that the synchronized transmission does not occur. In particular, in case that a signal transmitted in a contention based transmission mode is a signal transmitted via RACH, the synchronization between the user equipment and the Node B is not completed yet. So, since a transmission timing point of the user equipment is calculated from a downlink frame time, it is unable to guarantee the synchronization when the Node receives the signal. Yet, after an initial transmission on RACH, it can be assumed that the reception synchronization between the user equipments is completed in a contention based transmission mode used after the beginning of a timing control for each of the user equipments.
In the conventional W-CDMA or CDMA system, an orthogonal code channel dedicated to a signaling message necessary to be transmitted to a Node B from a user equipment exists. If a traffic channel is not established yet since a traffic does not exist between a user equipment and a Node B for a considerable time, a different transmission channel should be used to send an urgent or normal signaling message to the Node B from the user equipment instead of using in-band type signaling via the traffic channel. So, there exists a dedicated orthogonal channel exists to be used for this purpose. A representative example of this kind of signaling message is a request message. In particular, the request message is needed to notify the Node B that a traffic channel needs to be urgently scheduled since a size of transmitting buffer of the user equipment exceeds a specific level under the circumstance that there exist data to be transmitted by the user equipment.
As mentioned in the foregoing description, the uplink transmission modes of the LTE include the scheduled transmission mode and the contention based transmission mode. Due to the characteristics of the orthogonal multiplexing transmission, it is difficult to assign a dedicated channel to transmit a signaling message for user equipments in idle mode. In particular, if a dedicated channel for a transmission of a signaling message is assigned to all user equipments in idle mode, uplink resources are excessively wasted.